The Metal Oxide Semiconductor Field Effect Transistor (“MOSFET”) is a well known type of semiconductor transistor that may be used as a switching device. A MOSFET is a three terminal device that includes a source region and a drain region that are separated by a channel region, and a gate electrode that is disposed adjacent the channel region. A MOSFET may be turned on or off by applying a gate bias voltage to the gate electrode. When a MOSFET is turned on by application of a gate bias voltage that is greater than or equal to a threshold voltage, current is conducted through the channel region of the MOSFET between the source region and the drain region. When the bias voltage is removed from the gate electrode (or reduced below the threshold voltage level), the current ceases to conduct through the channel region. By way of example, an n-type MOSFET has n-type source and drain regions and a p-type channel. An n-type MOSFET may be designed to turn on when a gate bias voltage is applied to the gate electrode that is sufficient to create a conductive n-type inversion layer in the p-type channel region that electrically connects the n-type source and drain regions, thereby allowing for majority carrier conduction therebetween. MOSFETs may also be designed as “normally-on” devices that are turned off by application of a gate bias voltage to the gate electrode.
The gate electrode of a MOSFET is typically separated from the channel region by a thin gate insulating pattern, such as a silicon oxide pattern. Because the gate electrode of the MOSFET is insulated from the channel region by the gate insulating pattern, minimal gate current is required to maintain the MOSFET in its on-state or to switch the MOSFET between its on-state and its off-state. The gate current is kept small during switching because the gate forms a capacitor with the channel region. Thus, only minimal charging and discharging current is required during switching, allowing for less complex gate drive circuitry.
There is an increasing demand for high power MOSFETs that can pass large currents (e.g., tens of Amps or more) in their “on” state and block large voltages (e.g., hundreds or even thousands of volts) in their reverse blocking state. In order to support high current densities and block such high voltages, power MOSFETs typically have a vertical structure with the source and drain on opposite sides of a thick semiconductor layer structure. In very high power applications, power MOSFETs are typically formed in wide band-gap semiconductor material systems (herein, the term “wide band-gap semiconductor” encompasses any semiconductor having a band-gap of at least 1.4 eV) such as, for example, silicon carbide (“SiC”), which has a number of advantageous characteristics including, for example, a high electric field breakdown strength, high thermal conductivity, high electron mobility, high melting point and high-saturated electron drift velocity. Relative to devices formed in other semiconductor materials such as, for example, silicon, electronic devices formed in silicon carbide may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under higher radiation densities.